The invention relates to nuclear medicine, and more particularly relates to Single Photon Emission Computed Tomography ("SPECT") nuclear medicine studies. In its most immediate sense, the invention relates to attenuation correction of cardiac SPECT studies.
In a conventional SPECT (Single Photon Emission Computed Tomography) study of an organ such as the heart, a radioisotope (Tc-99m, Tl-201, for example) is administered to the patient and the radioisotope is taken up by the heart muscles. Then, the patient is placed in a scintillation camera system and one or more scintillation camera detectors are rotated about the long axis of the patient. These detectors pick up gamma radiation that leaves the patient, and the resulting data is used to form three-dimensional images ("SPECT images" or "tomographic images") of the distribution of the radioisotope within the patient.
Such three dimensional SPECT images can be calculated based on a set of two-dimensional images ("projections" or "projection images") acquired by the scintillation camera system; this calculation process is known as image reconstruction. The most commonly employed method of image reconstruction is known as "filtered backprojection". When filtered backprojection reconstruction is used to reconstruct SPECT images from scintigraphic projection images obtained from a scintillation camera, some well-known distortions introduce errors ("artifacts") in the result. One of the most important distortions is caused by attenuation of gamma radiation in tissue.
As a consequence of attenuation, image values in the various projections do not represent line integrals of the radioisotope distribution within the body. It is therefore necessary to correct for this, and the process for doing so in SPECT is known as attenuation correction.
Many techniques for attenuation correction in SPECT assume that the linear attenuation coefficient of the body is uniform and impose such uniformity as a mathematical constraint in the image reconstruction process. However, for a very important class of studies, namely cardiac SPECT studies, the linear attenuation coefficient of the body is in fact highly nonuniform. This is because lung tissue has a lower attenuation than do, e.g., the blood and other non-lung tissue.
Thus, in SPECT studies of, e.g., the heart, a SPECT reconstruction of the image of radioactivity within the heart will necessarily contain artifacts caused by the unequal attenuation coefficients of, e.g., the lungs and the body. Such artifacts also appear in SPECT cardiac images taken from obese patients and from large-breasted female patients.
It is known to measure the actual attenuation coefficients of body tissues by placing a line source of gamma radiation on one side of the body and measuring the transmission of the gamma radiation through the body as a function of direction, i.e. collecting transmission CT data, as the line source is scanned across the patient's body. However, existing line sources, and existing scintillation camera systems that use them, suffer from certain disadvantages.
One such disadvantage is that existing scanning line sources are mechanically awkward. Another such disadvantage is that it is very difficult to collect sufficient transmission CT data while at the same time not overburdening the camera. Still another such disadvantage is that it is difficult to calibrate the activity of the line source so as to produce a higher radiation density for use with obese patients or large breasted female patient and a lower radiation density for use with thin patients.